Inductively coupled plasmas ("ICPs") generated with radio frequency ("RF") waves having a frequency generally between 1 MHz and 100 MHz are capable of providing charged particle (electron and ion) concentrations in excess of 10.sup.11 cm.sup.-3 and ion currents to wafer substrates in excess of 5 mA/cm.sup.2. The ICP source is thus competitive with electron cyclotron resonance ("ECR") plasma sources for plasma processing applications in integrated circuit manufacturing processes. Inductively coupled RF plasma sources have advantages over both capacitively coupled RF plasma sources and ECR plasma sources.
In contrast to capacitive RF coupling, inductively coupled RF plasmas have substantially lower intrinsic plasma potentials (&lt;50 V) and achieve a substantially higher ionization efficiency (&gt;5%). Also, the intrinsic plasma potential is relatively independent of the RF power. The low intrinsic plasma potential is useful in applications where high ion energies cannot be tolerated.
As in the case of ECR systems, the ion energy of an inductively coupled RF plasma can be varied independently by biasing the integrated circuit wafer with a separate RF power supply. The ICP source, however, has the advantage of operating over a pressure range that is more compatible with process requirements (1 mTorr to 50 mTorr). An ECR source is most effective at pressures below 10 mTorr. In addition, the ICP source can provide a larger diameter (15 cm to 30 cm), homogeneous plasma, in a compact design, and at substantially lower cost than an ECR source. Since the operating pressure is higher, the pumping requirements for a given gas flow rate are more modest.
A first type of prior plasma source employing RF induction coupling, couples energy into the plasma through whistler or helicon waves. This source is called a helicon plasma source. In the presence of a magnetic field ranging from 100 G to 1 kG directed along the axis of the source, a standing whistler wave can be excited by applying an RF voltage to a loop antenna located around the source cavity. Although these axial magnetic fields are generally weaker than the magnetic fields employed in ECR sources, the plasma is non-uniform across the diameter of the source. Thus, the wafer must be located away or "downstream" of the source, in a region where the plasma is sufficiently uniform. This requires the input power of the source to be increased to maintain a sufficient plasma density (i.e., electron and ion concentration) at the downstream position. Also, large solenoidal coils are required to generate the axial magnetic field. These increase source cost and complexity.
A second type of prior plasma source differs from the generic whistler wave or helicon source by omitting the axial magnetic field. The wafer can therefore be placed within the plasma generation region. Even though the peak plasma densities (5.times.10.sup.11 cm.sup.-3) for such a source are about an order of magnitude lower than those for the whistler wave source, the proximity of the wafer to the source ensures that processing rates are comparable. Etch rates of over 1 .mu.m/min are possible for many materials of interest. This source is simpler, more compact, and cheaper than the helicon plasma source.
The second type of induction plasma source employs a multi-turn pancake coil located along the top surface of a cylindrical vacuum chamber. A quartz vacuum window, typically 0.5 in. thick, isolates the coil from the chamber. When the coil is powered by an RF source, large currents circulate in the coils. These currents induce intense electric fields inside the chamber that sustain the plasma.
The time-varying magnetic and electric fields generated by a pancake coil are proportional to the coil current, and scale as the square of the number of coil turns. The uniformity of the induced field improves with increasing coil turns. However, the inductance of the coil is proportional to the square of the number of coil turns. This implies that the voltage drop across the coil increases with an increasing number of coil turns for a fixed coil current. As an example, the voltage drop across a 5 .mu.H coil for an RMS current of 20 A at 13.56 MHz is 8.5 kV. Such a high voltage is a hazard and results in capacitive energy coupling between the coil and the plasma. Capacitive coupling is undesirable because the intrinsic plasma potential increases dramatically if a significant amount of energy is transferred via capacitive coupling. These issues constrain the number of coil turns to about three in prior RF plasma sources.
Therefore a need has arisen for an RF plasma source which minimizes the number of system components, efficiently uses output power, provides good plasma uniformity, and maintains coil voltages at safe levels.